Traffic service providers now broadcast traffic and travel information through radio transmission to help drivers to safely and efficiently travel. Although such services are limited today to city areas and along major freeways, it is expected that such traffic information service will cover most of the entire country in the near future. A typical example of such a traffic information service is RDS-TMC (Radio Data System-Traffic Message Channel) which transmits traffic and travel information of event, status and location to receivers via FM transmission.
Since the RDS-TMC services provide useful information or traffic and travel, an RDS-TMC receiver is preferably incorporated in a vehicle in combination with a navigation system. As is known in the art, a navigation system calculates an efficient route to a destination and guides a driver to the destination through the calculated route. In such a case, typically, an audio/video system of a vehicle is used to produce sounds and images for the RDS-TMC receiver and the navigation system.
FIG. 1 is a schematic diagram showing an example of structure of such a system incorporating the RDS-TMC receiver and the navigation system. In FIG. 1, a head unit 40 is an audio/video system (multimedia terminal) of a vehicle which typically includes an AM/FM radio, CD/DVD player, an LCD monitor screen, etc. A navigation system 50 is connected, typically through cables, to the head unit 40 to display the various functions of the navigation system 50, such as various menus, map images, a calculated route, icons, etc. The navigation system 50 includes a GPS receiver 52 which receives GPS satellite signals through GPS antenna A6 to determine the current position of the vehicle in combination with gyroscopes and speed sensors mounted on the vehicle.
An RDS-TMC receiver 30a is to receive the traffic and travel information contained in an RDS-TMC signal from an RDS-TMC service provider (broadcaster) 10 through wireless transmission. Typically, the RDS-TMC signal is an FM (frequency modulated) signal whose frequency is assigned to a predefined FM channel. As to which particular FM channel is assigned to a particular RDS-TMC signal may vary depending on a particular location, service provider, surrounding communication environment, etc.
The RDS-TMC receiver 30a receives the RDS-TMC signal at an antenna A1 and converts (demodulates) the RDS-TMC signal to an intermediate frequency (IF) signal. The intermediate frequency signal includes coded messages regarding traffic information which is specific to a particular region where the user is located. In the example of FIG. 1, the RDS-TMC receiver 30a decodes the traffic and travel information (RDS-TMC data) and sends the decoded RDS-TMC data to the head unit 40 of the vehicle through a cable 38. Alternatively, the RDS-TMC receiver 30a modulates the decoded RDS-TMC data to produce an FM signal again by an modulator. The FM signal having the decoded traffic and travel information (RDS-TMC data) is transmitted by an antenna A3 of the RDS-TMC receiver 30a to the head unit 40 through an antenna A5.
The head unit 40 produces voice messages or video images indicating the traffic and travel information. The arrangement of the RDS-TMC receiver 30a which is connected to the head unit 40 by the cable 38 is simple although it requires to physically connect the cable 38 therebetween. The arrangement of the RDS-TMC receiver 30a which produces the FM signal to be received by the head unit 40 through the antenna is convenient because a user can enjoy the traffic and travel information service by simply carrying-on the RDS-TMC receiver 30a without physically connecting to the head unit 40.
FIG. 2 is a schematic diagram showing an example of structure of a system incorporating an FM modulator in the vehicle. This example shows the case where an FM modulator 30b is used in the vehicle to reproduce the audio sounds by the head unit 40 of the vehicle. The FM modulator 30b receives FM modulated audio signals, etc. from a portable audio device 20 such as an IPOD™ (a portable digital audio/music player by Apple Computer, Inc., California) on a cradle 25 through wireless communication (between antennas A1 and A2). Alternatively, the FM modulator 30b receives audio signals, etc. from the audio device 20 or other device directly through wired communication by connecting a cable 28 therebetween. Examples of other device for use with the FM modulator 30b include a cellular phone, audio book, hand-held computer, PDA, etc.
The FM modulator 30b frequency-modulates the audio signals to transmit the FM modulated audio signals through an antenna A4. The FM modulated audio signals are received by the head unit 40 through the antenna A5 so that a user can enjoy music, audio sounds, etc., by the head unit 40 of the vehicle. The arrangement of FIG. 2 is convenient because a user can enjoy the favorite music, etc., by simply placing the FM modulator 30b in the vehicle without need of physically connecting the audio device 20 (IPOD™) and the FM modulator 30b to the head unit 40.
FIG. 3 is a schematic diagram showing an example of a receiver system 30c incorporating both the RDS-TMC receiver and the FM modulator in the vehicle. This example shows the case where either one of the RDS-TMC receiver or FM modulator or both thereof are used in combination with the head unit 40 of the vehicle. Each of the RDS-TMC receiver and the FM modulator in the receiver system 30c functions in the same manner as described with reference to FIGS. 1 and 2, respectively.
FIGS. 4A and 4B are schematic diagrams showing an example of FM channel arrangement and problems associated with the case where both the RDS-TMC receiver and the FM modulator are operated at the same time. In FIGS. 4A and 4B, FM channels fm7-fm14 are illustrated in which a bandwidth d between the two adjacent channels is 200 kHz. As is well known in the art, such FM channels are established between 88 MHz and 108 MHz in USA. The example of FIGS. 4A and 4B shows the case where the RDS-TMC signals are broadcasted by the service provider 10 through the FM channel fm10 while the FM signals transmitted by the FM modulator for the audio data is in the FM channel fm11.
In other words, the FM channels (frequency bands) for the RDS-TMC transmission and the FM audio transmission are very close to one another. In the worst case scenario, the same channel such as fm10 may be used for both the RDS-TMC transmission and the FM audio transmission. In such a situation, interference may arise between the two signals depending on various conditions, for example, power levels of the signals, surrounding topological situations such as buildings and mountains, weather conditions, etc. Typically, such interference tends to occur between the FM signal from the service provider to be received by the RDS-TMC receiver and the FM signal generated by the FM modulator to be received by the head unit 40. This is because the power level of the FM signal from the output of the FM modulator is much larger than that of the RDS-TMC signal at the input of the RDS-TMC receiver.
FIG. 4B shows such a problem in which frequency spectrum of the FM signal for the RDS-TMC data and frequency the FM signal for the IPOD™ (audio device) audio data are deteriorated because of the interference between the two FM signals. Thus, the traffic and travel information may not be sufficiently extracted from the FM signal from the RDS-TMC service provider. Since it is usually necessary to prioritize the traffic and travel information over the audio sounds, etc., there is a need to more securely receive the RDS-TMC signal when both the RDS-TMC receiver and the FM modulator are used at the same time.